VLSM

1.1.1  What is VLSM and why is it used?

As IP subnets have grown, administrators have looked for ways to use their address space more efficiently. This page introduces a technique called VLSM. With VLSM, a network administrator can use a long mask on networks with few hosts, and a short mask on subnets with many hosts. -

In order to implement VLSM, a network administrator must use a routing protocol that supports it. Cisco routers support VLSM with Open Shortest Path First (OSPF), Integrated IS-IS, Enhanced Interior Gateway Routing Protocol (EIGRP), RIP v2, and static routing.

VLSM allows an organization to use more than one subnet mask within the same network address space. VLSM implementation maximizes address efficiency, and is often referred to as subnetting a subnet.

Classful routing protocols require that a single network use the same subnet mask. As an example, a network with an address of 192.168.187.0 can use just one subnet mask, such as 255.255.255.0.

A routing protocol that allows VLSM gives the network administrator freedom to use different subnet masks for networks within a single autonomous system.  Figure shows an example of how a network administrator can use a 30-bit mask for network connections, a 24-bit mask for user networks, and even a 22-bit mask for networks with up to 1000 users.

The next page will discuss network address schemes.